I have been "big" since I was 18, and hit 18.5 stone (dress size 24) after the birth of my last child. Being slim / losing weight / hating being overweight dominated my thoughts. Everything I did was coloured by my obesity, I thought about my weight at least 50 times a day.
I joined the gym and started calorie counting. I started to see results quite quickly, which I found highly motivating. I felt happier and was proud of what I was achieving and despite my focus on exercise and what I was eating, I spent far less time thinking about my weight than I had previously.
Unfortunately, after around 6 months (and nearly 3 stone down), I became very ill. Due to my illness, the weight fell off me, eventually I was hospitalised weighing 5 stone 9lbs. I was seriously ill, suffering from malnutrition and picked up every bug going. I was thin - far thinner than I had ever imagined or wanted to be - but there was no joy in buying size 6 clothes. My skin hung off me, like I was a clothes hanger with a sheet draped over it. My cheekbones were super sharp, my eyes were sunken into the sockets and you could clearly see my skull - I really did look just like a skeleton.
Thanks to the wonderful NHS, I was eventually able to eat and to digest food again, so started to be able to put some weight back on. My stomach had shrunk, so it took far less food for me to feel full and I had a whole new attitude towards food and my body.
I have been very overweight and I have been very underweight. When I was big, I thought about being thin ALL the time. When I was tiny, I hoped I wouldn't die, ALL the time. I've now been between 10.5 and 11 stone (size 14/16) for the last couple of years. I've got boobs, a big bum and a bit of a tummy, but I rarely think about my weight these days. I think about making sure I'm eating healthily enough for my wellbeing and being a good example of a positive body image for my DC.
In answer to the OP, I'm happier somewhere in the middle and I should have spent less time thinking about my weight - I've learned to be careful what you wish for!